It’s Always Sunny in Philadelphia Season 6 Episode 7 (It’s Always Sunny in Philadelphia s06e07) with title “Who Got Dee Pregnant” aired on Thursday, October 28th 2010 at 10:00 pm via FX Network.
Here is summary for It’s Always Sunny in Philadelphia Season 6 Episode 7 :

Dee’s pregnant, and she tells the guys that one of them made her that way during a Halloween party that none of them can remember. So they try to piece the evening together, “Potemkin”-style, aided by Artemis (Artemis Pebdani)